Molded plastic containers, more specifically containers molded from organic polymer material, have been available in numerous shapes and for a variety of purposes ranging from egg cartons and hamburger packs to seedling containers and lightbulb packages. Particularly when the container is formed of expanded organic polymer material such as expanded polystyrene, the containers possess heat insulation and cushioning properties which make them very desirable for the foregoing uses. However, when such containers are made in an open hollow shape, substantially beyond the small size of a hamburger pack or egg carton, the inherent flexibility of the material works to its disadvantage and the containers generally have insufficient rigidity to be useful in such larger size ranges. Such disadvantage is particularly noted in containers which are used for carrying potted plants or the like. Such containers generally require a substantial internal volume compared to wall thickness and the loose soil used in potting is capable of exerting pressures in all directions with the result that a simple container constructed of expanded organic polymer material too readily gives way.
Additionally, containers for potted plants require one or more drainage openings in the bottom wall. The provision of such openings does not lend itself well to the continuous formation of the containers. In the usual method of forming a container of expanded polymer material, a wide, continuous sheet of plastic material is heated to soften it for molding, and the sheet is advanced between the parts of a multi-cavity mold. The mold parts are pressed together to form the containers while they are cooled to fix the polymer material in its molded shape, whereupon the mold parts are separated and the process is repeated indefinitely. While it is a simple matter to design the mold die surfaces so that a portion of the sheet material is punched out during mold formation, to form drainage openings, the vacuum cleaning system required for such operation is often inadequate for removing all waste material. Such material is often left over in the mold cavity and then is entrained in the outer surface of the next container molded in that cavity, marring that container in appearance by the presence of the waste material embedded in its surface. As a result, an inordinate number of rejects is experienced when openings are attempted to be made in the same operation that molds the container. Accordingly, a subsequent punching step is required. However, such a step necessitates feeding the molded carton from the molding station to a punching station where they are registered with punching equipment that includes male and female dies actuated to punch out the openings at the desired location in each carton. The slugs of punched out material must then be vacuumed away and the cartons fed to a cutting and trimming station that cuts them out of the large sheets and separates them for stacking. The hole punching equipment adds appreciably to the cost of the basic machine and also considerably increases the size and weight of the machine. Precisely adjusted equipment is required for registering the cartons accurately with the hole punching equipment so that the openings are properly spaced. Hole punching equipment causes many problems and a significant percentage of cartons must be scrapped because of various malfunctions of this accessory. Similarly, the cutting and trimming station adds appreciably to the cost of the basic machinery and requires additional precision registration equipment.
The present invention provides molding equipment and a container design which overcomes the foregoing difficulties and disadvantages. A container design is provided which has sufficient inherent rigidity so that expanded organic polymer material, such as expanded polystyrene, can be used in the construction of a relatively large size container for potted plants. Additionally, molding equipment is provided which enables the ready formation of such a container which combines molding, drainage hole "punching" and container cutting in one mold operation. The present invention eliminates a hole punching station and eliminates a cutting station to provide a fully formed potted plant container with drainage openings and of rigid construction, in a single operation.
Specifically, a one-piece container is provided molded from organic polymer material. The container has an integral side wall structure, a bottom wall molded integral with the side wall structure and an upper peripheral solid lip entirely around and extending outwardly from the side wall structure and molded integral therewith. The lip provides rigidity and strength to the container by being formed with a horizontally extending flat lower surface and an upper surface convexly curved from the outer edge of the lip to the side wall structure. More specifically, the upper lip surface curves from the flat lower lip surface to the inner surface of the side wall structure. The side wall structure can be cylindrical or oval in cross section, or in a preferred embodiment as illustrated herein, can be formed of four side walls, each tapering outwardly from the bottom wall so that one container is stackable within another container.
Drainage openings are formed through the bottom wall by separating, without removing, a portion of the bottom wall and extending it outwardly from the remaining portion. This is accomplished by limited vertical shearing during the molding operation, as will hereinafter be described and enables the formation of a drainage aperture without any waste material. The mold is designed so as to raise the region of the bottom wall adjacent the drainage opening sufficiently above the remaining portions of the bottom wall so that the drainage opening is disposed a pre-set distance above a supporting surface. The material immediately adjacent the drainage opening is compressed together completely therearound to strengthen that region.
The mold utilized to provide the present structure includes male and female mold parts having their die surfaces in confronting relation and shaped to contour the surfaces of the sheet to form the container. The mold parts are spaced one from the other a distance sufficient to permit the insertion of a sheet or organic polymer material between the die surfaces which are formed with confronting annular regions for defining a peripheral lip for the container. The container is cut and trimmed during the molding operation by means of a steel ruled trim die. The trim die is in the form of a strap of serrated steel slidably disposed entirely around the male mold part with its serrates pointing downwardly and closely adjacent the outer edge of the male mold part. As the male and female mold parts are moved together to form a container, the steel strap is slid downwardly so that the serrates tear through the sheet of organic material to define the outer edge of the container lip. The serrated steel strap is supported by a plate which is mounted for reciprocal movement over the male mold part and spring loaded to a position spaced vertically from the male mold part. As the plate is pushed downwardly during the molding operation, it pushes the male mold part into the female mold part against the sheet material and simultaneously the serrated steel strap moves downwardly to cut the container from the sheet material.
The male mold part is formed with an opening into which is slidably disposed a sharp-edged hole former. The hole former is connected to the reciprocal plate through the opening in the male mold part and shears a drainage opening through the sheet material upon reciprocation of the plate. The movement of the plate is limited to a distance sufficient to form an opening through the sheet material but insufficient to entirely remove the material that is sheared away. Rather, the material is merely extended outwardly from the bottom of the formed container. In a specific embodiment, the leading edge of the hole former is discontinuously shaped so as to form the aperture by separating a plurality of portions of the bottom container wall and extending those separated portions away from the adjacent region of the bottom wall.